pupilsparing

Pupil sparing, or pupil-sparing, is a clinical descriptor used in neurology and ophthalmology to describe a pattern in oculomotor disorders in which the pupil remains normal in size and reactive to light. It is most commonly discussed in relation to cranial nerve III palsy. This finding contrasts with pupil-involving third nerve palsy, where the pupil is dilated and nonreactive, often suggesting compression from an aneurysm or other mass.

Pathophysiology and causes: The oculomotor nerve has somatic fibers that control most eye muscles and parasympathetic fibers that control the pupil. Microvascular ischemia (for example, related to diabetes or hypertension) tends to spare the outer parasympathetic fibers, producing a pupil-sparing palsy. In contrast, compressive lesions, such as a posterior communicating artery aneurysm, typically affect the superficial parasympathetic fibers first and produce a dilated, nonreactive pupil.

Clinical significance: The presence or absence of pupil involvement helps guide initial diagnostic thinking. Pupil-sparing third nerve palsy raises suspicion for microvascular ischemia, but does not exclude other serious causes. In adults with acute onset diplopia or ptosis, clinicians may still pursue imaging if red flags are present or if vascular risk factors are not explanatory.

Evaluation and management: A thorough neurologic and ophthalmic examination is essential, including assessment of ptosis, eye position, and pupillary responses. Management includes addressing underlying risk factors (e.g., diabetes, hypertension) and, depending on presentation, targeted imaging with MRI/MRA or CT angiography to assess for aneurysm or other lesions.

Prognosis: Ischemic, pupil-sparing third nerve palsy often improves over weeks to months with appropriate risk-factor management, while lesions causing pupil involvement require different urgent interventions.

See also: third nerve palsy, anisocoria, neuro-ophthalmology.